This invention relates to powder metallurgy and particularly to the consolidation of metal powders into nearly solid metals.
Although this operation is performed successfully in commercial operations by the simultaneously application of both heat and pressure, it is a costly operation. The process as practiced commercially, is entitled "hot isostatic pressing", and is referred to by the acronym HIP. In the HIP process, the powders are encased in metal containers of the desired configuration which are welded closed after evacuation of the container air. The containers, or "cans" as they are colloquially known, are placed in an autoclave designed for very high pressures and high temperatures to achieve densification of the contained powders. This procedure produces compacts with densities greater than 95% of theoretical and is technically satisfactory. It suffers from the previously mentioned high cost, from the inconvenience of canning and decanning, from the need for the skilled welders required to weld perfectly the thin metal of the can, the limited commercial HIP facilities existing in the nation, and finally the chamber size limitations of the few available HIP facilities. The latter two disadvantages, as well as the high cost, are functions of the difficulty of achieving the desired strength levels and creep resistance in metal structures designed to bear high levels of force or pressure while being operated at high temperatures.
One method to offset the difficulties associated with the HIP process is to compact powders at ambient temperatures, followed by vacuum sintering. This is known as CIP-Sinter and the equipment required is less costly. Although this procedure is successful in some cases, there are instances where it is not. For example, if the powders being compacted do not deform at ambient, there is no resultant mechanical interlocking of deformed particles and the resultant compact is too weak for subsequent handling. The process is then not applicable.
Another version of a technique to avoid HIP operations is the CAP.RTM. process, or "consolidation by atmospheric pressure". This process, as detailed in U.S. Pat. No. 4,227,927, entitled "Powder Metallurgy," which issued to Herbert L. Black et al on Oct. 14, 1980, can produce near theoretical density. In this process the powder is enclosed within an evacuated glass container, which is then embedded within a free flowing refractory powder, such as graphite, and heated within an air atmosphere furnace. Although this process may function well for materials whose diffusion constants are high at usable sintering temperatures, it functions inadequately if the migratory capability of the metal or material atoms is low and little diffusion occurs. In such an instance one might logically reconsider the HIP process to achieve adequate densification of a compact.
Given the difficulties associated with simultaneous application of heat and pressure, as in HIP, and the limitations of pressure applied before heating, as in CIP-Sinter, or heating with an atmospheric pressure differential, as in CAP.RTM., there remains the desire to achieve HIP level of results in an easier and less costly manner for hard, nondeformable powders with low diffusivity capability.